Important considerations in designing an A/D converter are speed, component count, and resolution. Flash converters provide the greatest speed. To convert an analog input voltage into an n-bit digital output code, a flash converter usually has 2.sup.n -1 input
comparators that compare the input voltage with 2.sup.n -1 corresponding reference voltages supplied from a resistive voltage divider. For example, see J. Peterson, "A Monolithic Video A/D Converter", IEEE JSSC, Dec. 1979, pp. 932-937.
The principal disadvantage of the flash converter is a high component count due to the large number of input comparators. A large chip area is needed to implement the device in integrated circuit form. Numerous schemes have been proposed to cut the number of comparators. For example, see U.S. Pat. Nos. 4,270,118 and 4,386,339. These schemes normally accept a loss in conversion speed as a compromise.
A "folding" system is one of the more promising techniques for reducing component count. In a folding A/D converter, a set of input amplifiers respond to the input voltage and a corresponding set of reference voltages in such a way as to generate one or more pairs of complementary waveforms that have a repetitive rounded triangular shape as a function of the input voltage. A group of fine comparators convert these sawtooth waveforms into a string of bits which are encoded into the least significant bits of the output code. The most significant bits are supplied from a group of coarse comparators which operate on the input voltage along a separate channel from the folding array. See R. van de Plassche et al, "A High-Speed 7 Bit A/D Converter," IEEE JSSC, Dec. 1979, pp. 938-943. Also see R. van de Grift et al, "A Monolithic 8-Bit Video A/D Converter," IEEE JSSC, June 1984, pp. 374-378.
The chip area for a folding converter is reduced dramatically because it utilizes considerably fewer comparators than an otherwise equivalent flash converter. While folding systems do offer relatively good speed with low power dissipation, the inherent "rounding off" of the tips of the repetitive triangular waveforms must be taken into account to avoid loss in resolution. It is highly desirable to have a simple technique that takes maximum advantage of the linear portions of these waveforms.